Why do the builders not make a flashing first and then a seal over that?
Much like the flashing on a house chimmney.
Take some narrow alluminum flashing, form it to fit, then put clear RTV sealant on the back of it and attach it to the cab and camper with short small sheet metal screws or even pop rivets from your local hardware store. Then after that is in place, put the rubber sealent over that.
Just my thought on how to reseal it if it were mine.